Optical fiber with low loss and nanoscale structurally homogeneous core

ABSTRACT

An optical fiber has a core region that is doped with one or more viscosity-reducing dopants in respective amounts that are configured, such that, in a Raman spectrum with a frequency shift of approximately 600 cm −1 , the fiber has a nanoscale structure having an integrated D2 line defect intensity of less than 0.025. Alternatively, the core region is doped with one or more viscosity-reducing dopants in respective amounts that are configured such that the fiber has a residual axial compressive stress with a stress magnitude of more than 20 MPa and a stress radial extent between 2 and 7 times the core radius. 
     According to another aspect of the invention a majority of the optical propagation through the fiber is supported by an identified group of fiber regions comprising the core region and one or more adjacent cladding regions. The fiber regions are doped with one or more viscosity-reducing dopants in respective amounts and radial positions that are configured to achieve viscosity matching among the fiber regions in the identified group.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 15/086,169, filed on Mar. 31, 2016.

U.S. patent application Ser. No. 15/086,169, filed on Mar. 31, 2016,claims the priority benefit of U.S. Provisional Patent Application Ser.No. 62/196,465, filed on Jul. 24, 2015.

The above applications are owned by the assignee of the presentapplication, and are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to the field of fiber optics,and in particular to an improved optical fiber with low loss and ananoscale structurally homogeneous core.

Background Art

Due to the rapid spread of Internet services, the demand is increasingexponentially for low-loss optical fibers that are capable oftransmitting large amounts of data over long distances.

Core homogeneity has a significant impact on optical transmissionperformance. Current production fibers have cores that are doped withgermanium (Ge) and typically have an attenuation that is above 0.18dB/km. Such high attenuation is due to scattering from Ge-dopant as wellas nanoscale crystalline defects. While fibers with a Ge-free core canremove one source of attenuation, it is important to reduce thenanoscale crystalline defect concentration in the fiber core to furtherreduce attenuation to below 0.18 dB/km.

It is possible to reduce the concentration of nanoscale crystallinedefects by lowering the speed at which the fiber is drawn. However, theuse of a lower draw speed increases the amount of time required tofabricate a fiber, resulting in increased manufacturing costs.

SUMMARY OF INVENTION

These and other issues are addressed by aspects of the presentinvention, aspects of which are directed to an optical fiber with lowloss and a nanoscale structurally homogeneous core.

According to one aspect of the invention, an optical fiber comprises aplurality of concentric fiber regions, including a core region andsurrounding cladding regions, wherein the concentric fiber regions aredoped with one or more index-modifying dopants in respective amounts andradial positions that are configured to create a selected index profile.The core region is doped with one or more viscosity-reducing dopants inrespective amounts that are configured, such that, in a Raman spectrumwith a frequency shift of approximately 600 cm⁻¹, the fiber has ananoscale structure having an integrated D2 line defect intensity ofless than 0.025.

According to a further aspect of the invention, the core region is dopedwith one or more viscosity-reducing dopants in respective amounts thatare configured such that the fiber has a residual axial compressivestress with a stress magnitude of more than 20 MPa and a stress radialextent between 2 and 7 times the core radius.

According to another aspect of the invention a majority of the opticalpropagation through the fiber is supported by an identified group offiber regions comprising the core region and one or more adjacentcladding regions. The fiber regions are doped with one or moreviscosity-reducing dopants in respective amounts and radial positionsthat are configured to achieve viscosity matching among the fiberregions in the identified group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an isometric view of an exemplary step-index optical fiber.

FIG. 2 shows a refractive index profile for the fiber shown in FIG. 1.

FIG. 3 shows an isometric view of a substrate tube having an innercircumference onto which have been deposited a series of layers ofchemical soot, in accordance with a modified chemical vapor depositiontechnique.

FIG. 4 shows an isometric view of a preform resulting from the sinteringand consolidation of the substrate tube shown in FIG. 3.

FIG. 5 shows a diagram illustrating the drawing of a fiber from thepreform shown in FIG. 4.

FIG. 6 is a three-dimensional schematic of an exemplary silica molecularnetwork.

FIGS. 7 and 8 show Raman spectra measured in two groups of fibers: afirst group having cores doped with halogen dopants, and a second grouphaving cores doped with both halogen and alkali dopants.

FIG. 9 shows a table showing the integrated D2 line intensity in the twogroups of fibers of FIGS. 7 and 8.

FIG. 10 shows a graph illustrating the general correlation between fiberattenuation at 1550 nm and D2 defect line intensity.

FIG. 11 shows a table that lists the ionic radius and charge for alkalisand halogens.

FIGS. 12 and 13 show potassium and chlorine dopant concentration versuspreform radial position for two exemplary preforms containing relativelylow K-dopant concentration.

FIGS. 14 and 15 show potassium and chlorine dopant concentration versuspreform radial position for two exemplary preforms containing relativelyhigh K-dopant concentration.

FIG. 16 shows a micrograph of a preform sample, for which the chlorineand potassium dopant concentrations are plotted in FIG. 14.

FIG. 17A shows refractive index as a function of radial position in analkali-free fiber.

FIG. 17B shows the fiber's axial stress profile as a function of radialposition.

FIG. 18A shows refractive index as a function of radial position inwhich K-dopant was deposited in the center of the preform from which thefiber was drawn. FIG. 18B shows the fiber's axial stress profile as afunction of radial position.

FIG. 19 shows the spectral loss of a fiber doping with potassium in thecore region resulting in a reduced concentration of nanoscalecrystalline defects; and

FIGS. 20 and 21 show, respectively, side and front views an opticalfiber according to a further aspect of the invention.

FIG. 22 shows an index profile for the fiber shown in FIGS. 20 and 21.

FIG. 23 shows a table setting forth a number of parameters and dopingschemes for the fiber shown in FIGS. 20 and 21.

FIG. 24 shows a table that sets forth exemplary core doping schemes inaccordance with various aspects of the invention.

FIGS. 25 and 26 show an example of Raman spectra deconvolution intoindividual Gaussian, Lorentzian and polynomial spectral components todetermine the D2 defect line intensity.

DETAILED DESCRIPTION

The invention is directed to low-loss optical fibers that display a highdegree of core homogeneity, and techniques for manufacturing suchfibers. Exemplary practices of the invention, described below, providefiber designs that can achieve an attenuation of 0.18 dB/km or lower at1550 nm, even when drawn at speeds of 3 meters per second or greater.

According to an aspect of the invention, the above fiber characteristicsare obtained by doping the core region of a fiber preform according to adopant scheme that includes one or more viscosity-reducing dopants. In anumber of practices of the invention described below, an alkali is usedas a viscosity-reducing dopant. As discussed below, theviscosity-reducing properties of an alkali dopant added only to the coreregion of a fiber preform extend beyond the core region, and result in asignificant reduction in the formation of nanocrystalline defects as thepreform is drawn into fiber.

FIG. 1 shows a diagram of an exemplary step-index optical fiber 100,which provides context for the present discussion. It will beappreciated that the techniques described herein are applicable to otherfiber designs.

Fiber 100 is fabricated from silica (SiO₂) to which chemical dopantshave been added to create a plurality of concentric regions, each ofwhich is characterized by a respective refractive index. Fiber 100comprises a core region 101, an inner trench region 102, a shoulderregion 103, and an outer cladding region 104.

FIG. 2 shows an exemplary refractive index profile 110 for fiber 100,showing an up-doped core 111, down-doped trenches 112, up-dopedshoulders 113, and down-doped outer cladding region 114. It is notedthat in a step-index fiber design, refractive index is commonlyexpressed using the fractional refractive index difference Δn_(region):

${\Delta\; n_{region}} = \frac{n_{region} - n_{0}}{n_{region}}$where n_(region) is the refractive index of a given fiber region, and n₀is a reference refractive index (typically the refractive index ofundoped silica).

For the purposes of the present discussion, it is assumed that amodified chemical vapor deposition (MCVD) technique is used to fabricatefiber 100. However, it will be apparent that the aspects of theinvention described herein are generally applicable to other fibermanufacturing techniques.

In an MCVD technique, a large-diameter silica-based substrate tube isloaded into a lathe and is continuously rotated around its longitudinalaxis. As the tube is rotated, layers of chemical soot are deposited ontothe tube's inner circumference. Each preform region (corresponding to arespective fiber region) is built layer by layer, with chemical dopantsadded as required.

FIG. 3 shows an isometric diagram of an MCVD preform 120, prior tosintering and consolidation. Preform regions 121, 122, and 123,corresponding respectively to the core 101, inner trench 102, andshoulder 103 of fiber 100, have been built layer by layer onto the innercircumference of substrate tube 124, which corresponds to the outercladding 104 of fiber 100. It is noted that preform region 123 is builtfirst and that the core region of the preform 121 is built last.

After all the preform regions have been formed, the substrate tube isheated to cause the soot layers to sinter, and to cause the tube andsoot layers to be fused and consolidated into a cylindrical preform 130,shown in FIG. 4. As illustrated schematically in FIG. 5, the preform 130is then loaded into a draw tower and drawn into an optical fiber 140having a core and surrounding regions that correspond to preform regions131, 132, 133, and 134 (FIG. 4) (and that also correspond to sootregions 121, 122, 123 and substrate tube 124 in FIG. 3). As indicated byweight symbol 141 and arrow 142, a controlled tension is applied to thefiber 140 as it is being drawn.

As discussed below, the fiber drawing process illustrated in FIG. 5,when used in conjunction with prior doping schemes, results in arelatively high concentration of nanocrystalline defects in the drawnfiber, which in turn leads to a relatively high level of attenuation.

Aspects of the present invention are directed to doping schemes thatresult in an optical fiber having the following characteristics:

-   -   (a) a nanoscale structurally homogenous core;    -   (b) an extensive compressive axial stress in the fiber core and        adjacent regions; and    -   (c) viscosity matching among the fiber's radial regions        supporting a large percentage of optical propagation through the        fiber.

The theoretical basis of the invention is set forth below, followed by adescription of a number of exemplary practices of the invention.

FIG. 6 is a three-dimensional schematic of an exemplary silica molecularnetwork 150. Nanocrystalline defects result from the formation of 4-foldand 3-fold ring structures 151, 152 within the otherwise amorphousnetwork. These ring structural defect structures were reported by A. E.Geissberger and F. L. Galeener in “Raman studies of vitreous SiO₂ versusfictive temperature,” Phys. Rev. B, vol. 28, no. 6, pp. 3266-3271(1983), and illustrated in FIG. 1 from Vukelic et al., “StructuralModification of Amorphous Fused Silica Under Femtosecond LaserIrradiation,” ASME 2008 International Manufacturing Science andEngineering Conference, vol. 1, Evanston, Ill., USA, Oct. 7-10, 2008.

Within these ring structure, the O—Si—O bond angles (ϕ) areapproximately 102°, which is smaller than the bond angles ofapproximately 109° in the near-perfect SiO₄ tetrahedrons. The distortionof the SiO₄ tetrahedrons to form the smaller O—Si—O bond angles resultsfrom strain energy that is introduced during the draw process and thatis stored in the defect structures as the fiber cools.

The presence of nanocrystalline defects within an optical fiber can bequantified using Raman spectroscopy. A Raman spectrum measured in thecore of an optical fiber is relatively broad due to the amorphous natureof the silica network. Within this broad Raman spectrum, relativelysharp peaks are detected, indicative of nanocrystalline defects having ahigh strain energy within the amorphous network. Sharp D1 and D2“defect” lines at Raman shifts of approximately 500 cm⁻¹ and 600 cm⁻¹appear in the Raman spectra of bulk vitreous silica. The intensity ofthese defect lines increases with the concentration of thenanocrystalline defects.

Nanocrystalline defects form scattering centers in an optical fiber andincrease fiber attenuation. These structural defects also induce addedloss upon exposure to hydrogen or ambient radiation, as they are moreprone to color center formation. High-speed fiber draw, especially at alow draw temperature, further increases the number of these nanoscalecrystalline defects, resulting in a higher attenuation. Attempts toreduce such structural defects require fiber drawing at low speeds, butthis incurs a higher fiber manufacturing cost. Therefore, it isdesirable to reduce the concentration of nanoscale crystallinestructural defects, particularly in the fiber core.

Raman spectral measurements in an optical fiber provide directmeasurements of nanocrystalline defects at the fiber core (i.e., withinthe portion of the fiber that carries greater than 80% of the opticaltransmission power). The type and concentration of the nanocrystallinedefects are affected by the glass composition of the fiber core, as wellas processing conditions during preform fabrication (e.g., ambient andtemperature during densification and consolidation, including tubecollapse) and during fiber draw (e.g., draw tension, speed andtemperature). Therefore, Raman spectral measurements on thenanocrystalline defects provide the most relevant data on the opticalperformance of a transmission fiber.

FIGS. 7 and 8 show Raman spectra 160, 170 measured in 13 differentoptical fibers, each belonging to one of two groups: Group A, in whichthe fiber cores were doped with halogen dopants; and Group 3, in whichthe fiber cores were doped with both halogen and alkali dopants. FIG. 7shows both the D1 defect line 161 and the D2 defect line 162. FIG. 8focuses on the D2 defect line 172.

The Raman spectral measurements in FIGS. 7 and 8 show that typicalsilica-core fibers have relatively large D2 intensities. The D2 line inthe Raman spectra is caused by the symmetric stretching of the regularnanoscale crystalline planar three-fold ring defects imbedded within theamorphous network.

In FIG. 7, Raman scattering intensities in different fibers weremeasured versus the frequency downward shift upon excitation at awavelength of 1.55 μm. Raman spectra from different fibers werenormalized against their respective peak heights at the ω₁ main linearound 440 cm⁻¹.

FIG. 8 shows the Raman spectra between frequency shifts between 515 cm⁻¹and 815 cm⁻¹ in order to more clearly show the intensity levels aroundthe D2 defect line. In FIG. 8, it can be seen that, at the D2 defectline, there is a readily identifiable difference between the respectiveintensities of the fibers in Group A (i.e., the upper group of traces171A) and the intensities of the fibers in Group B (i.e., the lowergroup of traces 171B). The difference between the two groups of fibersis indicative of the different concentrations of nanocrystalline defectsin these fibers. Thus, for the purposes of the present discussion, theconcentration of nanocrystalline defects in a given fiber is quantifiedusing the intensity level at the D2 defect line in that fiber's Ramanspectrum.

FIG. 9 is a table 180 showing the integrated D2 line intensity in thefibers in Group A and Group B. (The technique used to quantify theintegrated D2 line intensities, and therefore the respectiveconcentrations of nanoscale crystalline core defects, is described belowin the section entitled “Methodology.”)

The Group A fibers, containing halogen core dopants, have an integratedD2 line intensity of 0.04. The Group B fibers, containing both alkaliand halogen core dopants, have an integrated D2 line intensity of0.022-0.026. Thus, the integrated D2 line intensity of the Group Bfibers is approximately 60% (±5%) of the integrated D2 Line intensity ofthe Group A fibers.

FIG. 10 shows a graph 190, in which plot 191 illustrates the generalcorrelation between fiber attenuation at 1550 nm and D2 defect lineintensity. It is observed that fibers having a structurally homogeneouscore (e.g., the fibers in Group B) tend to have lower fiber attenuation.

The nanoscale structurally homogenous core in Group B fibers is achievedby adding alkali and halogen co-dopants into the fiber core, where morethan 70% of optical power propagates. Because of the increasedhomogeneity of their cores, the group B fibers display a substantiallyreduced D2 line intensity, as shown in FIG. 8.

Alkali and halogen dopants in the fiber core have an ionic chargevalence of +1 and −1 respectively. These charged alkali and halogendopants disrupt the planar three-fold ring structures by relieving thestrain energy and restoring the O—Si—O bond angles to the higher bondangles very similar to those in the stable tetrahedron network inamorphous silica. The bond angle relaxation relieves the strain energywithin the planar defects resulting in lower scattering and thereforelower attenuation of optical signals propagating in the fiber core.

In addition to their ionic charge, the ionic size of the alkali andhalogen dopants also plays a role in removing the nanoscale crystallinedefects.

FIG. 11 shows a table 200 that lists the ionic radius and charge foralkalis and halogens. Table 200 lists the “crystal” ionic radii, whichcorresponds more closely to the physical size of ions in a solid.

A small quantity (e.g. <4,000 ppm) of most alkali can be introduced intosilica without causing devitrification (i.e., crystallization). Of thehalogens, only fluorine and chlorine can be readily doped in silica insignificant concentrations.

Based on the matching ionic radii of the alkali and halogen co-doping,it is preferable to co-dope the following alkali-halogen combinations ina silica core to remove nanocrystalline defects:

(1) potassium (K)+chlorine (Cl)

(2) rubidium (Rb)+chlorine (Cl)

(3) sodium (Na)+fluorine (F)

While this suggests that approximately equal molar concentrations ofalkali and halogen dopants are preferable for charge balance, an excesshalogen dopant concentration can be beneficial in reducing glassviscosity and in increasing the glass relaxation rates to achieve alower energy state with less atomic fluctuations, and therefore lowerscattering losses.

According to an aspect of the invention, a fiber core is co-doped with asignificant chlorine concentration and with a suitable alkali co-dopant.(As set forth above, suitable alkali co-dopants for chlorine includepotassium and rubidium.) Much higher chlorine co-doping levels (greaterthan 500 ppm, and preferably more than 1,000 ppm) can be usedbeneficially together alkali dopant in reducing fiber attenuation.

Chlorine dopant can be used to enhance fiber design flexibility versusthe use of fluorine dopant. It is because the chlorine doping at thesilica fiber core increases the refractive index whereas fluorine dopingdecreases the core index. In particular, 10,500 ppm chlorine increasesthe silica refractive index by 0.001 Δn and 3,450 ppm fluorine dopantdecreases the silica index by 0.001 Δn. For a given cladding refractiveindex, chlorine doping increases the optical power within the coreregion and can therefore be used, for example, to reduce themacrobending losses. There are many such examples well known to thefiber designer in which it is desirable for a dopant to increase therefractive index of the glass. Likewise, in some instances it isdesirable to use F to improve the structural homogeneity of the glass,such as in regions with reduced refractive index.

Furthermore, chlorine doping, especially by means of oxygen-free SiCl₄,has additional advantages in dehydration and purification. Firstly, theaggressiveness of SiCl₄ reaction with SiO₂ surface molecules enables theincorporation of high levels of chlorine concentrations into the silicamatrix. This also yields an advantage in purification of OH, transitionmetals, and other contaminants. A typical reaction might be written in aform to emphasize the oxidation product SiO₂:4(O_(1.5)Si—O_(0.5(surface)))+SiCl₄→4(1.5OSiCl)+SiO₂

In addition, the advantage of higher chlorine incorporation will furtherassist purification by reacting with OH, impurity metalloids, and metaloxides, to form chlorides. These chlorides have significant volatility.Therefore, the use of oxygen-free SiCl₄ doping enhances thewell-developed purification methods of chlorine purification of silica,as well as incorporating higher levels of chlorine within the silicathat can be used to advantageously modify refractive index profiles.

The reaction of SiCl₄ with impurity metal oxides and metalloid oxides(the most common form of impurities in a silica matrix formed byoxidation) is substitutional with respect to the oxide. Formation of thevery stable SiO₂ yields a thermodynamic advantage in almost all cases.Examples are:Metalloid: GeO₂+SiCl₄→GeCl₄+SiO₂Metal: Fe₂O₃+1.5SiCl₄→2FeCl₃+1.5 SiO₂

A similar advantage is gained by dehydration which is observable at roomtemperature with SiCl₄, while molecular Cl₂ is not effective until >800°C. Thus, a SiCl₄-doped silica core made by sintering the un-doped silicasoot in an oxygen-free SiCl₄ ambient can result in very low OH contentin the resultant fiber; and the OH peak in such fibers can be less than0.33 dB/km at a wavelength of 1385 nm.

The overall dehydration reaction using SiCl₄ is given as:4(O_(1.5)Si—OH(surface))+SiCl₄+5SiO₂+4HCl.

As discussed below in the “methodology” section, the alkali dopantconcentration can be characterized by the peak concentration in thefiber core center or the concentration radially averaged within the coreradius; and such concentration quantities reported in the fiber can beestimated from the measurements made in the preform using experimentallydetermined scale factors, 1/βi.

As further discussed below, it is more appropriate to characterize thehalogen dopant concentration by the concentration radially averagedwithin the fiber core radius since the halogen concentration may nothave the maximum value at the preform and fiber core center.Furthermore, the average dopant concentration reported in the fiber canbe estimated from the measurements reported in the preform usingexperimentally determined scale factors, 1/βi.

FIGS. 12 and 13 are a pair of graphs 210, 220 that show potassium andchlorine dopant concentration versus preform radial position for twoexemplary preforms containing relatively low K-dopant concentration(i.e., less than 200 ppm peak concentration). In each graph 210, 220 thepotassium concentration is plotted on the right vertical scale and thechlorine concentration on the left scale.

FIGS. 14 and 15 are a pair of graphs 230, 240 that show potassium andchlorine dopant concentration versus preform radial position for twoexemplary preforms containing relatively high K-dopant concentration(i.e., more than 3,000 ppm peak concentration). In each graph, thepotassium concentration is plotted on the right vertical scale and thechlorine concentration on the left scale.

In a fiber according to the present invention, the desirable upperlimits on the alkali and halogen dopant concentration are determined bysilica devitrification, which increases scattering losses.

It is interesting to note the effects of potassium and chlorineconcentrations on silica devitrification. FIG. 16 shows a micrograph 250of a preform sample, for which the chlorine and potassium dopantconcentrations are plotted in FIG. 14. While this sample has about 3,400ppm K₂O peak concentration at the core center, the central region 251remains transparent without devitrification mainly because chlorine atthe same region was depleted to <100 ppm by the potassium dopingprocess. However, the clear central region 252 is surrounded by adevitrified region 252, which appears as a dark ring because of lightscattering. In the devitrified region, the potassium concentration isapproximately 2,000 ppm while the chlorine concentration is aboutapproximately 800 ppm. This observation on silica devitrificationestablishes the upper limit of the potassium doping concentration atabout 2,000 ppm. This upper limit is increased by reducing the chlorineconcentration.

Experimental data shows that alkali dopant makes a significant change inthe residual axial stress in the fiber core and adjacent regions whenthe preform is drawn into fiber.

FIG. 17A is a graph 260 in which trace 261 shows refractive index as afunction of radial position in an alkali-free fiber. FIG. 17B is a graph270 in which trace 271 shows the fiber's axial stress profile as afunction of radial position.

FIG. 18A is a graph 280 in which traces 281 and 282 show refractiveindex as a function of radial position in a pair of fibers in whichK-dopant was deposited in the center of the preform from which the fiberwas drawn. Traces 281 and 282 show, respectively, the refractive indexprofile of a first fibers drawn at an applied tension of 34 gm and asecond fiber drawn at an applied tension of 122 gm. FIG. 18B is a graph290, in which traces 291 and 292 show the axial stress profile as afunction of radial position for the first and second fibers.

As shown in FIG. 17B, a tensile (i.e., positive) stress is found in thecore region of an alkali-free fiber. As shown in FIG. 18B, a compressive(i.e., negative) stress is found in an alkali-doped fiber. Moresignificantly, the radial width of the compressive stress profile in analkali-doped fiber extends much further than the originally doped regionupon scaling from the preform dimension.

As shown in FIG. 18B, in K-doped fibers the axial compressive stressextends to a radius of approximately 25 μm, or approximately 5 times thecore radius in both a fiber drawn at 34 gm and the same fiber drawn at122 gm. The radial extension of the compressive stress fibers arises asa result of dopant diffusion mainly during fiber draw.

During the transformation process from preform to fiber, the relativeradial extent of alkali dopant distribution increases about 5 times.Therefore, it is expected that the alkali concentration in the drawnfiber is approximately 25 times less than that in the preform.

As discussed below in the “Methodology” section, β≈25 for potassium.Consequently, the potassium concentration in fibers drawn from preformsshown in FIG. 12-15 can be 25 times lower than those measured in thepreforms. The concentration and radial extend of K doping in the fiber,as with any dopant, depends on the degree of diffusion in the glass andis therefore affected by thermal history, initial doping spatial profilein the preform and glass composition.

Furthermore, a residual axial compressive stress with a sufficientmagnitude more than 20 MPa together with the stress radial extentpreferably 1.2 times, and more preferably 2 times the core radiusresults in good structural homogeneity in the core and adjacent regionscritical for optical transmission performance because a significantoptical power propagate in this radial range. The lower limit in theradial extent of the compressive stress is defined by the need to ensuremost of the optical power propagates in the structurally homogeneous andtherefore low attenuation regions. Since different fiber designs havedifferent spatial power distributions, the compressive stress profileshould have a lower radial limit within which more than 99%, andpreferably more than 99.9%, of the total optical power propagates.

The upper limit in the radial extent of the compressive stress isdetermined by several factors. Firstly, there is no significantadvantage in reducing the optical attenuation by extending thecompressive stress region to the radial region in which little opticalpower propagates. Secondly, the axial compressive stress region shouldnot extend too widely that is indicative of too much alkali dilution dueto alkali diffusion to a large radius during fiber processing. Thirdly,it is known that after the draw tension is released, axial stress, σz,equilibrium is established throughout the fiber cross section such thatthe axial compressive stress in the core central region must be balancedby the axial tensile stress in the outer cladding region, i.e.

∫₀^(R)σ_(z)rdr = ∫₀^(R₁)σ_(z)rdr + ∫_(R₁)^(R)σ_(z)rdr = 0where compressive stress is between r=0 and r=R1; and tensile stress isbetween r=R1 and r=R=cladding radius.

By increasing R1 and for a given compressive stress within r<R1, it isexpected that a higher tensile stress at the outer cladding resulting ina lower mechanical strength of the optical fiber. This considerationalso imposes an upper limit on the radial extent of the axialcompressive stress region.

FIG. 19 is a graph 300 that shows the spectral loss of a fiber dopedwith potassium in the core region, resulting in a reduced concentrationof nanoscale crystalline defects. The fiber has an attenuation of 0.16dB/km at 1550 nm, and an attenuation of 0.159 dB/km between 1560 and1580 nm.

Due to the aliovalency of phosphorus (P³⁺ or P⁵⁺) relative to silicon(Si⁴⁺) and its ionic radius (58 pm for P³⁺, and 52 pm for P⁵⁺) matchingrather closely with the 54 pm ionic radius of Si⁴⁺, phosphorus dopantaddition in the fiber core also reduces the concentration of nanoscalecrystalline defects as indicated by a lower D2 line intensity. However,since phosphorus oxide has an infrared absorption edge at a shorterwavelength than silica, a high phosphorus dopant concentrationintroduces added loss at communication wavelengths, especially between1.3 μm and 1.55 μm. Therefore, phosphorus dopant concentration between0.2% and 2% is the optimal balance between the D2 defect reduction andthe attenuation increase from the infrared loss edge.

It will be seen that viscosity-matching techniques according to aspectsof the invention exploit the fact that alkali dopants reduce the silicaviscosity over a large radial range from the initial doping location.For example, FIGS. 18A-B show that the compressive axial stressextending to about 5 times the fiber core radius even though potassiumwas deposited only within the preform core region.

FIGS. 20-23 are a series of diagrams of an optical fiber 310 accordingto a further aspect of the invention. FIGS. 20 and 21 show,respectively, side and front views of the fiber 310; FIG. 22 shows anexemplary index profile 320 for the fiber; and FIG. 23 shows a table 330setting forth for each fiber region the following parameters: outerradius, index difference, viscosity ranking prior to alkali doping, afirst exemplary alkali doping scheme; and a second exemplary alkalidoping scheme.

Fiber 310 comprises five concentric regions layered on top of eachother: a core region 311A, a pedestal region 311B, an inner trench 311C,an outer trench 311D, and an outer cladding 311E (corresponding to thefollowing regions of index profile 320: up-doped core 321A, eitherundoped or slightly doped pedestal 321B, down-doped inner trench 321C,down-doped outer trench 321D, and cladding 321E). According to an aspectof the invention, a low attenuation is achieved by using alkali dopingto create viscosity matching, during fiber draw, among the fiber'sradial regions 311A-311E.

Prior to alkali doping:

Region 311A contains a non-alkali dopant (e.g., a halogen), whichreduces its viscosity (to rank 3), and increases its core index.

In an exemplary design, region 311B does not contain much dopant, andhas a higher viscosity (rank 5) than adjacent regions 311A and 311C. Inalternative designs, region 311B is doped with a halogen (e.g.,fluorine) that reduces the refractive index and also silica viscosityprior to alkali doping.

Regions 311C (viscosity rank 1), 311D (viscosity rank 2) and 311E(viscosity rank 3) contain different concentrations of F-dopants. Silicaviscosity decreases monotonically with a higher F-concentration.

In a first alkali doping scheme:

Region 311B is doped with a significant concentration of alkali,resulting in a lower viscosity to match those in Regions 311A and 311C.(The viscosity-reducing effects of alkali doping are most pronounced inRegion 311B, which has a high viscosity prior to doping. However, alkalidoping is less effective in further reducing the viscosity of Regions311C and 311A, which have already been softened from the presence ofother dopants.)

In those practices of the invention in which region 311B alreadycontains halogen doping (e.g., fluorine) prior to alkali doping, a loweralkali doping concentration will be needed to achieve viscositymatching.

Region 311A contains a small amount of alkali by intentional doping andby alkali diffusion during preform processing and fiber draw.

Region 311C contains a trace amount of alkali mainly by alkali diffusionduring fiber draw.

Regions 311D and 311E do not have much alkali doping. (Viscositymatching during fiber draw between Regions 311D and 311C does not havemuch impact on attenuation and on other optical properties because verylittle optical power propagates at the interface between these regions.)

In a second exemplary alkali doping scheme:

Regions 311A and 311B are doped with a significant amount of alkali,resulting in a lower viscosity to match the viscosity of Region 311C.

In those practices of the invention in which region 311B alreadycontains halogen doping (e.g., fluorine) prior to alkali doping, a loweralkali doping concentration will be needed to achieve viscositymatching.

Region 311C contains a trace amount of alkali, mainly resulting fromalkali diffusion during fiber draw.

Regions 311D and 311E do not have much alkali doping.

FIG. 24 shows a table that sets forth a number of exemplary core dopingschemes in accordance with aspects of the invention discussed above.

In exemplary doping scheme 1, a fiber core is doped with fluorine (0 ppmto 150,000 ppm), chlorine (0 ppm to 15,000 ppm), and phosphorus (0.2% to2%)

In exemplary doping scheme 2, a fiber core is doped with potassium (5ppm to 2,000 ppm) chlorine (100 ppm to 15,000 ppm), and phosphorus (0%to 2%).

In exemplary doping scheme 3, a fiber core is doped with rubidium (5 ppmto 2,000 ppm), chlorine (100 ppm to 15,000 ppm), and phosphorus (0% to2%).

In exemplary doping scheme 4, a fiber core is doped with sodium (5 ppmto 2,000 ppm), fluorine (100 ppm to 15,000 ppm), and phosphorus (0% to2%).

Methodology

Quantification of the D3 Defect Line Intensity

There is now described the methodology used to quantify the D2 defectline intensity.

The intensity of D2 defect line can be quantified by de-convoluting theRaman spectrum measured around the D2 frequency shift into theindividual Gaussian, Lorentzian and polynomial spectral components.

FIGS. 25 and 26 are a pair of graphs 350, 360 showing an example ofRaman spectra deconvolution into individual Gaussian, Lorentzian andpolynomial spectral components to determine the D2 defect lineintensity. The D2 line intensity is given by the integrated intensity ofthe line shape best-matching the D2 frequency shift.

Alternately, the intensity of the D2 defects can be quantified by

δ I = ∫_(f₁)^(f₂)[I(f) − I_(o)(f)]dfwhere I(f) is the intensity of Raman scattering at frequency shift, f;and I(f) is the scattering intensity extrapolated from the adjacentfrequency regions to give the background Raman scattering intensity. Theintegration interval, f₂−f₁, is the range of frequency shift in whichI(f)>I_(O)(f) in the integrand of the above equation.

In order to provide a consistent measure of the D2 line defectintensity, the above δ1 value is normalized with respect to the broadRaman spectrum of the same fiber.

Quantification of Dopant Concentration

Because a preform has an outer diameter that is much larger than that ofa fiber drawn from the preform, dopant concentration measurements in apreform typically give more accurate data than measurements in the drawnfiber. The average dopant concentration of a given dopant is conservedbecause dopants are deposited near the preform center, and because atall conceivable thermal histories the deposited dopants do neatevaporate from the preform or fiber.

In particular, the average concentration of a given dopant in thepreform is same as that in the fiber, i.e.:

${\int_{0}^{R_{1}}{\frac{C_{i}\left( r_{1} \right)}{R_{1}^{2}}r_{1}{dr}_{1}}} = {\int_{0}^{R_{2}}{\frac{C_{i}\left( r_{2} \right)}{R_{2}^{2}}r_{2}{dr}_{2}}}$where Ci is the concentration of dopant type i; r₁ and r₂ refer to theradial positions of the preform and fiber respectively; and R₁ and R₂are the preform and fiber clad radius respectively.

While the average dopant concentration throughout the fiber is same asthat throughout the preform, their dopant distributions can be verydifferent due to dopant diffusion. For optical fiber applications, ourprimary interest is the quality of the fiber core region through whichmost optical power propagates. The average dopant concentration withinthe fiber core radius, a, is most relevant in the fiber transmissionquality. The following equation shows that the dopant concentrationwithin the fiber core is related to the concentration within the preformcore radius R by a factor, 1/βi.

${\int_{0}^{a}{\frac{C_{i}\left( r_{1} \right)}{a^{2}}r_{1}{dr}_{1}}} = {\frac{1}{\beta_{i}}{\int_{0}^{R}{\frac{C_{i}\left( r_{2} \right)}{R^{2}}r_{2}{dr}_{2}}}}$

When dopant diffusion occurs for a very short time at a smalldiffusivity, it is expected that βi≈1. However, βi approaches a verylarge value after dopant diffusion for a very long time at a fastdiffusivity.

As shown in FIGS. 12-15, the peak alkali dopant concentration usuallyoccurs at the preform core center, r₂=0; and we expect the fiber corecenter, r₁=0, also has the maximum alkali dopant concentration. We notethat the peak optical power and the peak alkali dopant concentrationboth occur at the fiber core center. To the first approximation, thealkali dopant concentration at the fiber core center is related to thatat the preform core center by the following relation:

${C_{i}\left( {r_{i} = 0} \right)} \approx \frac{C_{i}\left( {r_{2} = 0} \right)}{\beta_{i}}$

Therefore, the alkali dopant concentration can be characterized byeither the average alkali concentration within the fiber core or thepeak alkali concentration at the fiber core center.

For a given dopant and fiber draw condition, the scale factor, 1/βi, canbe determined experimentally to correlate the dopant concentrationwithin the fiber core versus that measured within the preform core.

FIGS. 12-15 show that the peak halogen dopant concentration does notnecessarily occur at the preform core center, r₂=0, mainly because ofhalogen dopant depletion possibly by the consolidation ambient, e.g.oxygen. Since we cannot expect the maximum halogen dopant concentrationat the fiber core, it is more appropriate to characterize the halogendopant concentration by the average halogen concentration within thefiber core.

Conclusion

While the foregoing description includes details that will enable thoseskilled in the art to practice the invention, it should be recognizedthat the description is illustrative in nature and that manymodifications and variations thereof will be apparent to those skilledin the art having the benefit of these teachings. It is accordinglyintended that the invention herein be defined solely by the claimsappended hereto and that the claims be interpreted as broadly aspermitted by the prior art.

We claim:
 1. An optical fiber, comprising: a plurality of concentricfiber regions, including a core region and surrounding cladding regions,wherein the concentric fiber regions are doped with one or moreindex-modifying dopants in respective amounts and radial positions thatare configured to create a selected index profile with radial regionssupporting optical propagation through the fiber, wherein the coreregion is doped with viscosity-reducing dopants in respective amountsand radial positions that are configured to result in viscosity matchingamong the fiber's radial regions supporting optical propagation throughthe fiber, such that, in a Raman spectrum with a frequency shift ofapproximately 600 cm⁻¹, the fiber has a nanoscale structure having anintegrated D2 line defect intensity of less than 0.025.
 2. The opticalfiber of claim 1, wherein the core region is doped with one or moreviscosity-reducing dopants in respective amounts and radial positionsthat are configured such that the fiber has an attenuation of less than0.18 dB/km at 1550 nm.
 3. The optical fiber of claim 1, wherein the coreregion is doped with one or more viscosity-reducing dopants inrespective amounts and radial positions that are configured such thatthe fiber has an attenuation of less than 0.17 dB/km at 1550 nm.
 4. Theoptical fiber of claim 1, wherein the fiber is drawn at a speed ofgreater than 3 meters per second.
 5. The optical fiber of claim 1,wherein the core region is doped with amounts of P, Cl and F dopants,wherein the amount of P dopant is between 0.2% and 2%, wherein theamount of Cl dopant is between 0 and 15,000 ppm, and wherein the amountof F dopant is between 0 and 150,000 ppm.
 6. The optical fiber of claim1, wherein the core region is doped with amounts of alkali, halogen andphosphorus co-dopants.
 7. The optical fiber of claim 6, wherein the coreregion is doped with amounts of K, Cl, and P dopants, wherein the amountof K dopant is between 5 and 2,000 ppm, wherein the amount of Cl dopantis between 100 and 15,000 ppm, and wherein the amount of P dopant isbetween 0% and 2%.
 8. The optical fiber of claim 6, wherein the coreregion is doped with amounts of Rb, Cl and P dopants, wherein the amountof Rb dopant is between 5 and 2,000 ppm, wherein the amount of Cl dopantis between 100 and 15,000 ppm, and wherein the amount of P dopant isbetween 0% and 2%.
 9. The optical fiber of claim 6, wherein the coreregion is doped with amounts of Na, F, and P dopants, wherein the amountof Na dopant is between 5 and 2,000 ppm, wherein the amount of F dopantis between 100 and 15,000 ppm, and wherein the amount of P dopant isbetween 0% and 2%.
 10. An optical fiber, comprising: a plurality ofconcentric regions, including a core region and surrounding claddingregions that are doped with one or more index-modifying dopants inrespective amounts and radial positions that are configured to create aselected index profile, wherein the core region is doped with one ormore viscosity-reducing dopants in respective amounts and radialpositions that are configured such that the fiber has a residual axialcompressive stress with a stress magnitude of more than 20 MPa and astress radial extent between 2 and 7 times the core radius, and suchthat, in a Raman spectrum with a frequency shift of approximately 600cm⁻¹, the fiber has a nanoscale structure having an integrated D2 linedefect intensity of less than 0.025.
 11. The optical fiber of claim 10,wherein the core region is doped with one or more viscosity-reducingdopants in respective amounts that are configured such that the fiberhas an attenuation of less than 0.18 dB/km at 1550 nm.
 12. The opticalfiber of claim 10, wherein the core region is doped with one or moreviscosity-reducing dopants in respective amounts and radial positionsthat are configured, such that the fiber has an attenuation of less than0.17 dB/km at 1550 nm.
 13. The optical fiber of claim 10, wherein thefiber is drawn at a speed of greater than 3 meters per second.
 14. Theoptical fiber of claim 10, wherein the compressive stress has amagnitude and radial extent that are formed by an addition of alkali,halogen and phosphorus co-dopants in the core region of the fiber. 15.The optical fiber of claim 14, wherein the core region is doped withamounts of K, Cl, and P dopants, wherein the amount of K dopant isbetween 5 and 2,000 ppm, wherein the amount of Cl dopant is between 100and 15,000 ppm, and wherein the amount of P dopant is between 0% and 2%.16. The optical fiber of claim 14, wherein the core region is doped withamounts of Rb, Cl, and P dopants, wherein the amount of Rb dopant isbetween 5 and 2,000 ppm, wherein the amount of Cl dopant is between 100and 15,000 ppm, and wherein the amount of P dopant is between 0% and 2%.17. The optical fiber of claim 14, wherein the core region is doped withamounts of Na, F, and P dopants, wherein the amount of Na dopant isbetween 5 and 2,000 ppm, wherein the amount of F dopant is between 100and 15,000 ppm, and wherein the amount of P dopant is between 0% and 2%.